[Engineering] Proposal regarding the future of X-Wikimedia-Debug and testwiki

[Ori Livneh]

The X-Wikimedia-Debug header, for those of you who don't know, is an HTTP
request header that you can set on your requests (either manually, or by
using the Chrome[1] or Firefox[2] extensions). Requests bearing this header
are always treated as cache misses by Varnish, and they are always routed
to the same backend, mw1017.

In addition to handling X-Wikimedia-Debug requests, mw1017 is also
configured as the sole application server backend for all requests to
test.wikimedia.org. This was set up before X-Wikimedia-Debug existed, and
as a debugging tool it is (IMO) inferior to it, because X-Wikimedia-Debug
allows you to test code changes against any production wiki.

What I've seen happen before is developers (like me -- I've done this
before) live-hack code on mw1017 to debug some issue that is only showing
up in production. This can cause testwiki to break, which annoys developers
and editors who use testwiki for testing things like Lua modules or editing
functionality on mobile apps.

To reduce contention for mw1017, I propose that we do the following:

1) Keep testwiki, but don't special-case it in Varnish
(in other words, have testwiki requests go to the standard app server pool)

2) Reserve mw1017 exclusively for X-Wikimedia-Debug requests

3) Add a service alias (appservers-debug.svc.eqiad.wmnet) for mw1017 and
update the varnish backend config to use that, rather than hard-code mw1017
in VCL.

Thoughts?

 [1]:
https://chrome.google.com/webstore/detail/wikimediadebug/binmakecefompkjggiklgjenddjoifbb?hl=en-US
 [2]: https://addons.mozilla.org/en-US/firefox/addon/wikimedia-debug-header/
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